This invention relates to a process and materials for thermal imaging.
It has long been known that images can be formed by thermal imaging processes in which a donor sheet comprising a dye is placed adjacent a receiving sheet and selected portions of the donor sheet are heated to effect an imagewise transfer of the dye from the donor sheet to the receiving sheet, thereby forming the image on the receiving sheet. One such process is described in U.S. Pat. No. 2,616,961, issued Nov. 4, 1952; this patent notes that the heating of the donor sheet need not be effected by direct contact of the donor sheet with a hot object, but may be effected by exposing the donor sheet to radiant energy (for example, infra-red radiation) or corpuscular energy (for example, an electron beam). U.S. Pat. No. 3,147,377, issued Sept. 1, 1964, describes a similar process for production of color transparencies.
U.S. Pat. No. 3,924,041, issued Dec. 2, 1975, describes a heat-sensitive recording material comprising a first support, a transfer layer, and a second support on the opposed side of the transfer layer from the first support. The materials in these three layers are chosen such that before heating the adhesion strength between the transfer layer and the second support is smaller than the adhesion strength between the transfer layer and the first support, but, after heating to a temperature higher than the heat sensitive temperature of the transfer layer, the adhesion strength between the transfer layer and the second support becomes greater than the adhesion strength between the transfer layer and the first support. The transfer layer comprises, at least on the side in contact with the second support, a heat-sensitive composition containing as a major component a mixture of a heat-sensitive substance which is fluidized at a heat-sensitive temperature and an adhesiveness-imparting agent which can adhere to the second support at a temperature no higher than this heat-sensitive temperature.
Thermal imaging processes can be used for producing color images by successively superimposing a plurality of donor sheets over a single receiving sheet, with each donor sheet bearing a differently-colored dye, and heating only those portions of each donor sheet in which the corresponding color is required in the image. Typically, such color processes use three donor sheets providing yellow, cyan and magenta dyes, or four donor sheets providing yellow, cyan, magenta and black dyes. A process of the latter type is described in U.S. Pat. No. 4,803,496, issued Feb. 7, 1989; this patent describes adjustment of the area where the black ink is applied to prevent darkening of the image and resultant loss of color balance.
U.S. Pat. No. 4,587,198, issued May 6, 1986, describes a process for providing a color image comprising exposing a radiation sensitive layer over a vapor deposited colorant layer, and vaporizing the colorant to selectively transmit the colorant through the exposed layer. The change in solubility, permeability and/or crosslinking or polymerization of the exposed radiation sensitive layer causes differential migration of colorant through the exposed layer.
U.S. Pat. No. 4,602,263, issued July 22, 1986, describes a thermal imaging method for forming color images; this method relies upon the irreversible unimolecular fragmentation of one or more thermally unstable carbamate moieties of an organic compound to effect a visibly discernible color shift from colorless to colored or from one color to another.
U.S. Pat. No. 4,801,949, issued Jan. 31, 1989, describes a thermal imaging system in which a layer of rupturable capsules are formed on a sheet of paper, the coated sheet is exposed and the microcapsules are subjected to a uniform rupturing force, whereupon exposed microcapsules rupture and imagewise release chromogenic material contained within the capsules.
Recently, thermal transfer processes have been used commercially in printers intended for use as output devices for computers or other electronic data recording equipment, including cameras in which the image is recorded electronically on a magnetic medium. In such printers, the donor sheet is scanned by a thermal printing head having a plurality of small heating elements, so that the image on the receiving sheet is composed of a large number of dots each formed by one of the heating elements, in the same way that a conventional dot-matrix printer using an ink ribbon forms an image comprising a large number of ink dots. Such a thermal transfer printer is described in U.S. Pat. No. 4,855,758, issued Aug. 8, 1989; the printer described in this patent uses an electroconductive ink on the donor sheet and an electrode in physical contact with the donor sheet to prevent any path for electricity being formed between the donor sheet and the printing head.
U.S. Pat. No. 4,720,480, issued Jan. 19, 1988, describes donor and receiving sheets intended for use in such a thermal transfer printer. The face of the donor sheet which contacts the thermal printing head is provided with a heat-resistant slipping layer to prevent adhesion of the thermal printing head to the donor sheet. The receiving sheet comprises a base sheet, a receptive layer for receiving the dye transferred from the donor sheet, and an intermediate layer provided between these two layers, this intermediate layer having a low modulus of elasticity so that it becomes deformed during printing. The patent states that such deformation of the intermediate layer improves dye transfer from the donor sheet to the receiving sheet.
U.S. Pat. No. 4,755,396, issued July 5, 1988, describes an image receiving element for thermal printers, this element comprising a substrate bearing on at least one major surface thereof a coating of heat-sensitive material comprising a material capable of existing in a supercooled state after melting and subsequent cooling, at least one anti-fouling agent, and, optionally, a binder. The coating is stated to reduce the amount of material which fouls a thermal printing head contacting the image receiving element.
U.S. Pat. No. 4,555,427, issued Nov. 26, 1985, describes a receiving sheet for use in a thermal printing process, this receiving sheet having an image receiving layer comprising mutually independent islands of a first synthetic resin having a glass transition temperature of from -100.degree. to 20.degree. C. and having a polar group, and a second synthetic resin having a glass transition temperature of 40.degree. C. or above.
One of the problems in any thermal imaging process (or indeed in any process which relies upon the formation of an image by transfer of dye from a donor sheet to a receiving sheet, whatever method is used to effect such transfer) is ensuring sufficient transfer of the dye to produce an image of the requisite density on the receiving sheet. To assist dye transfer, and thus enhance image density, attempts have been made to provide the donor and/or receiving sheets with materials which assist in release of dye from the donor sheet or take-up of dye by the receiving sheet. For example, U.S. Pat. No. 3,088,028, issued Apr. 30, 1983, describes a heat duplicating system using a donor sheet having a heat-meltable coating. The receiving sheet (copy paper) used in this system can be provided on its image-receiving surface with a heat-modifiable, heat-softenable or low-melting solid, which when heated softens and becomes variously otherwise modified into a state in which it is a solvent for the dye.
U.S. Pat. No. 3,177,086, issued Apr. 6, 1965, describes a pressure-sensitive transfer in which the donor sheet ("transfer sheet") comprises a flexible foundation carrying a volatile, solvent-applied, heat-resistant frangible transfer layer substantially completely transferable to the receiving sheet ("master sheet"). The donor or receiving sheet may be coated with a film having an affinity when hot for both the receiving sheet and the transfer layer; this film is stated to effect a better, more complete transfer of the transfer layer after cooling and separation of the donor sheet from the receiving sheet.
U.S. Pat. No. 3,195,455, issued July 20, 1965, describes a thermal duplicating process in which the receiving sheet ("copy sheet") is coated with a film of a heat-meltable solid developer which when heated softens and becomes fluid and is thus converted into a solvent for the dye being transferred on to the receiving sheet.
U.S. Pat. No. 4,109,937, issued Aug. 29, 1978, describes a donor sheet for use in thermal imaging, this donor sheet comprising a substrate sheet having a coating comprising an organic acid which is volatilizable at thermal imaging temperatures, an additive consisting essentially of a fatty acid having from 10 to 26 carbon atoms or a metal salt thereof, and a polymeric binder compatible with the volatilizable acid. The presence of the additive is stated to control the physical nature of the acid layer and the subsequent volatility of the acid, thereby providing a composition which produces sharp, easily readable, permanent and dense images.
U.S. Pat. No. 4,321,404, issued Mar. 23, 1982, describes radiation curable coating compositions comprising polyfluorinated acrylates and methacrylates, polyethylenically unsaturated crosslinking agents and a film-forming organic polymer. There compositions are useful as release coatings in image transfer systems wherein a fused thermographic image is transferred from a release-coated surface to another surface.
U.S. Pat. No. 4,670,307, issued June 2, 1987, describes a thermal transfer recording sheet produced by placing, on one side of a sheet-like, heat-resistant substrate successively along the surface, one or more thermal transfer recording layers containing a recording material which contains a binder material and a coloring material and whose viscosity is lowered and controlled by temperature-raise recording control, so that transferability to recording medium is imparted, and a thermal transfer coating layer containing a hot-melt material which is miscible (compatible) with at least a part of the binder material. Thermal transfer recording using this sheet is effected by first subjecting the thermal transfer coating layer to temperature-raise recording control, forming a film of the hot-melt material on the surface of the recording medium at least on a portion to which the recording material is transferred, and conducting thereon thermal transfer recording as usual. The patent states that this reduces unevenness of transfer due to unevenness of the material receiving the coloring material, thereby enabling recording sensitivity to be improved.
Despite the efforts which have been made to improve dye transfer from a donor sheet to a receiving sheet, incompleteness and non-uniformity of dye transfer remain serious problems in thermal imaging. These problems are especially acute in thermal transfer printers, because of the brief contact time between the thermal printing head and any one pixel of the image, and because of the need to control closely not only the color but also the optical density of each pixel. For example, a 4 by 4 inch (102.times.102 mm.) image having a relatively low resolution of 100 dots per inch (about 4 dots per millimeter) contains 160,000 pixels of each color, or a total of 480,000 pixels for a three-color process. If such a print is to be produced in (say) two minutes using a print head containing 100 discrete heating elements, the contact time between a single heating element and each pixel cannot exceed 0.025 seconds. Even if only 16 levels of optical density of each color are required, it will readily be apparent that the requirements for speed and reproducibility of dye transfer in such a thermal imaging process are highly exacting. Furthermore, since any dye which cannot be transferred from the donor sheet to the receiving sheet within the brief contact time (even when the thermal printing head is set for maximum heating of a specific pixel) is effectively wasted, the lower the proportion of dye which can be transferred to the receiving sheet, the larger the amount of dye which must originally be present on the donor sheet, and the higher the cost of the donor sheet.
There is thus a need for a thermal imaging process which can achieve a high rate of dye transfer from a donor sheet to a receiving sheet, and the present invention provides such a process and materials for use therein.